hm... if u really like the girl... ask your cuz if s/he can give the girl's number to you... call her up, apologize for not going back and explain that the reason was because u were too shy, but that you really do like her. Then ask if she'd like to go to a movie, grab some coffee or lunch... basically, ask her out on a date.
Honesty is the best policy. If she likes you, then she'll forgive... if not... she'll blow you off... but either way, at least you'll know that you tried your best.
In the future, if you like a girl, just talk and be friendly. Don't have to be a super conversational ist, or the best anything else... just be yourself. Then, ask for her number.
Although I do agree with you that grabbing an unknown girl's butt is NOT a good idea... that's the best way to turn a girl off, get slapped, or go to jail... lol...
good luck, kid!